There is considerable interest in fluid density imaging of a wide variety of porous media, but the difficulty in acquiring a reliable density image with no relaxation time contrast is well known. Free induction decay (“FID”) based MRI methods appear to be ideally suitable to fluid content imaging, since the effective spin-spin relaxation time T2* is frequently well behaved, and single exponential, for realistic porous media.
Experiments on sedimentary rocks [1], [2] and on typical mortar and concrete samples [3] show that the FID rate
  (      1          T      2      *        )is dominated by the susceptibility difference between the pore fluid and solid matrix of the porous media sample, resulting in an effective single exponential T2* decay, that scales with B0. This occurs even when T2 (the spin-spin relaxation time) and T1 (the spin-lattice relaxation time) are multi-exponential due to the distribution of pore sizes [4].
Single point imaging (“SPI”) uses a pulse sequence first introduced by Emid and Creyghton [9] and extensively analyzed by Choi et al. [10]. SPI and SPRITE [5], an advanced SPI method, are FID based MRI methods which have proven to be ideal for imaging short relaxation time systems. The images acquired employing SPI methods are not distorted by artifacts due to B0 inhomogeneity, chemical shift and susceptibility variations [6]. The significant disadvantage is a long acquisition time, due to low flip angle RF pulses and the very limited number of FID points acquired after each excitation pulse [7]. Several modifications of SPI, focused on acquisition time reduction, have been published, including so called “multipoint k-space mapping” proposed by Cho and Ro [8]. However, with prior art FID based MRI methods, only a single data point is acquired in the presence of the gradient.